The V3 loop region of HIV-1 envelope glycoprotein has been shown to be critical for virus neutralization and contains an epitope known as the principal neutralizing determinant (PND). To determine whether neutralization of HIV-1 by human sera is mediated only by antibodies directed against variable region 3 (V3) epitopes, synthetic peptides representing the PND were used to measure binding antibodies in an ELISA, and as blocking agents in a standard neutralization assay. Using a series of peptides representing five strains of HIV-1 (SF2, HXB2, BRU, MN, Z3) neutralization blocking assays were run on a panel of sera selected on the basis of neutralizing and binding antibody titers. For the blocking assays, sera were preincubated with peptide for one hour prior to performing the neutralization assay. Each peptide was tested against their analogous virus in neutralization blocking assays. In addition, the MN peptide was tested against several other strains of virus. The results indicate that the exogenous MN peptide could block neutralization of HIV- 1MN, but not neutralization of other virus strains. The four other peptides did not demonstrate any appreciable blocking to date.